1. Field of the Invention
The present invention relates to an ink jet head for use in a printer and particularly, to a head for transferring ink to an ink discharge orifice while minimizing loss of a discharging pressure applied to the ink, an inject head for feeding ink without being affected by the pressure for ejection of the ink and minimizing the blockage with the ink, and a technique for forming such a head through sintering a ceramic or glass assembly.
The present invention also relates to a structure of a driver with a piezoelectric/electrostrictive element for use with such an ink jet head and a method of fabricating the same.
The present invention relates to an ink discharge opening provided in such an ink jet head.
2. Introduction to the Invention
Ink jet printers have widely been used as computer output devices. The ink jet printers are small in the overall size and low in the cost while reproducing high-quality printed images.
One of the key technologies of ink jet printers is a head. The action of a common ink jet head includes applying a pressure in a pressurizing chamber where a liquid ink for printing is stored and ejecting the ink in the form of droplets from a discharge orifice onto printing paper. Means for applying the pressure are mainly the displacement of a piezoelectric element and the pressurizing in an ink chamber by generation of tiny air bubbles with the use of a heater for ejection of ink. The former exhibits less consumption of power and thus is favorable for further reducing the overall dimensions.
Characteristic examples of the former model are disclosed in Japanese Patent Laid-open Publications (Heisei)6-40030 and (Heisei)8-238763.
As shown in FIGS. 18 and 19, a technique disclosed in Publication (Heisei)6-40030 has an ink orifice member 102 made of a plurality of layers joined to one over the other and provided with an ink discharge orifice 100 which is communicated at the rear to an ink pressurizing chamber 104. The ink pressurizing chamber 104 is defined by an ink pump member 112 which comprises a joint plate 108 having an opening 106 provided therein for communication and a spacer plate 110 which both are made of green layers of a ceramic material joined to one over the other and sintered together to form a solid and a closure plate 114 bonded to the back of the ink pump member 112.
A piezoelectric/electrostrictive element and a pair of electrodes are disposed on the outer surface of the closure plate 114. More particularly, the piezoelectric/electrostrictive element 146 is sandwiched between the lower electrode 114 and the upper electrode 142. Denoted by 116 is an ink feed chamber for feeding ink via an ink feed orifice 118 and the opening 120 to the pressurizing chamber 104. In that arrangement, the pressurizing chamber 104 is provided in the ceramic solid and exhibits higher sealing effects. However, the ink orifice member 102 comprises an ink discharge orifice plate 122 and an ink feed orifice plate 124 joined by a passage plate 126 to each other using an adhesive. The ink orifice member 102 is then bonded by the adhesive to the ink pump member 112. Accordingly, the sealing between the plates is not perfect and may thus cause leakage of the ink. Also, the ink discharge orifice plate 122 is made of a metallic material. Even if the bonding between the plates of different materials is negligible, the metallic material is clearly limited by a stress generated during drilling of the nozzle on decreasing the distance between openings and may hence interrupt the higher density processing.
A shock wave generated by the closure plate 114 is propagated throughout the ink and reflected on the plate 108 arranged in parallel with the oscillating closure plate 114, thus creating interference and diminishing its pressing force. The ink discharge orifice is designed to sharply become narrower towards the opening and may develop reflection of the shock wave thus declining the pressure.
FIG. 20 is a cross sectional view of an ink head disclosed in Publication (Heisei)8-238763. The ink head has an ink pump member 112 fabricated by a set of a closure plate 114, a spacer plate 110, an ink feed orifice plate 124, and a reinforcement plate 123 of a ceramic material joined and sintered to a solid form which is thus improved in the sealing effect as compared with the previous ink head disclosed in (Heisei)6-40030. However, the ink pump member 112 is not bonded by an adhesive to a passage plate 126 and an ink discharge orifice plate 122, hence being unfavorable in the sealing effect. As apparent, the conventional head fails to have a specific structure for propagating the shock wave to the nozzle without loss to effectively transfer the pressure for efficient ejection of ink.
The ink head shown in FIG. 18 has the ink feed orifice 118 arranged facing the closure plate 114 and thus receiving directly the shock wave, whereby the feed of ink will be disturbed. The ink in a common ink jet head of a printer is pressurized for producing a high-speed jet by the piezoelectric/electrostrictive action of a piezoelectric element. The pressure generated is propagated as a shock wave throughout the ink. When the shock wave runs in its forward direction and is reflected on the opposite wall of the ink pressurizing chamber, its reflection now moves in the opposite direction and thus interferes with the coming shock wave running in its forward direction, hence diminishing the pressing force and declining the ejection of the ink from the discharge orifice. As the pressurizing chamber of a conventional ink jet head in a printer has its bottom wall arranged in parallel with the oscillating side of the piezoelectric element, the shock wave from the oscillating side is propagated through the ink and reflected on the bottom wall to offset its pressing force thus decline the effectiveness of propagation of the pressure. Also, as the ink discharge orifice 100 is provided in a conical shape becoming narrower towards the opening, the shock wave creates more reflections on the inner wall of the conical orifice 100 and offsetting its energy. Such a conventional structure fails to avoid loss of the shock wave and decrease loss of the pressure.
While the ink heads are critically demanded for minimizing the overall dimensions and speeding up the ejection of ink, the shock wave has to be generated at higher frequency and its waveform will be acute. Its propagation hence depends largely on the shape of not only the pressurizing chamber but also the ink discharge orifice 100 which has to be carefully designed for not diminishing the energy of the shock wave. Both the ink heads shown in FIGS. 18 and 19 are unsuccessfully designed where the ink discharge orifice 100 becomes narrower towards the tip or the opening. This only contributes to the accumulation of ink to be discharged according to the Bernoulli theorem. If the ink is dried and solidified in the ink discharge orifice close to the ink pressurizing chamber during the printing action, its solids having a greater size than the diameter of the ink discharge orifice may be trapped thus blocking up the orifice.
The ink pressurizing chamber 104 is squared at each corner and may often trap air bubbles of which the elasticity diminishes the energy of the pressure. Also, as the ink jet heads are located in an array close to each other, the action of one may possibly be affected by the action of another.
The ink feed orifice 118 is also located to face against the forward direction of the shock wave and its ink feeding action may be interrupted by the shock wave. If the ink is dried while being in the stand-by state before the printing, its solids will hardly be removed. The oscillating action of one ink jet head may affect the action of another. It is however essential for every ink jet printer producing high density prints without reducing the printing speed to have a higher density type of the ink jet head. For the higher density action, the conventional heads are arranged with the ink discharge orifice provided in a metallic plate for minute piercing. However, such a piercing technique is limited to the diffusion of the stress created and the size of a jig employed and may hardly be suited for the high density processing.
The piezoelectric/electrostrictive element used as a driving source for ejection of the ink with the ink jet head is also not favorable in the efficiency. It is known that when the element is exposed to an electric field of a coercive level which develops in parallel with the direction of polarization of the element, it expands along the direction of the electric field and the polarization and retracts along a direction at a right angle to the direction. Also, when the electric field is reversed to develop opposite to the direction of the polarization, the element retracts along the direction and expands along a direction at a right angle to the direction. The (Poisson""s) ratio "sgr" of displacement along the direction at a right angle to the polarization direction to displacement along the direction of the polarization and the electric field is substantially up to 0.3 when the element is of a perovskite ceramic piezoelectric type such as using barium titanate. The ratio of displacement may not be very different in both the retraction and the expansion from that of a common PZT piezoelectric element made of a ceramic solid solution of PbZrO3 and PbTiO3.
The piezoelectric/electrostrictive element is generally located on the back of the ink jet head behind the ink pressurizing chamber and its oscillation energy is first transmitted to the wall of the ink pressurizing chamber which is thus oscillated to eject the ink.
FIG. 18 illustrates an example of the conventional ink jet head having a piezoelectric/electrostrictive element 146 located as a driver between an upper electrode 142 and a lower electrode 144 on the oscillator plate or closure plate 114 thereof.
When the voltage is applied between the lower electrode 144 at the positive and the upper electrode 142 at the negative, the electric field is developed upwardly. As the piezoelectric/electrostrictive element 146 is lengthwisely expanded and crosswisely retracted by the action of the electric field, its crosswise retracting movement of the element 146 is used for deflecting the oscillator plate 114 downwardly. When the ink pressurizing chamber 104 is pressed down, its ink discharge orifice 100 ejects a droplet of the ink. When not energized, the oscillator plate 114 remains in its horizontal state thus allowing the ink to be fed from the ink feed chamber 116 via the ink feed orifice 118 into the ink pressurizing chamber 104. In the conventional head, the oscillator plate or the closure plate 114 is deflected downwardly by the crosswise retracting movement of the element. As described above, the crosswise displacement is about ⅓ the lengthwise displacement and its efficiency will be low.
An ink jet head for an ink jet printer employing a bi-morphous type piezoelectric/electrostrictive element is disclosed in Publication (Heisei)8-118663 where the piezoelectric/electrostrictive element has a pair of PZT materials, which are opposite in the polarization to each other, sandwiched between two electrodes for increasing the electric distortion of the piezoelectric material.
In the above disclosed head, as best shown in FIG. 21, the bi-morphous type piezoelectric element 134 comprises a pair of piezoelectric/electrostrictive (PZT) elements 138 and 140, which are bonded to two, front and back, sides of an electrode member 136 and of which the directions of polarization are opposite to each other and orthogonal to the electrode surface, and a pair of driver electrodes 142 and 144 located on both sides of the paired elements and is located on an oscillator plate 132 made of a glass substrate 128 covered with a metallic layer 130 as above an ink pressurizing chamber 104. The paired piezoelectric/electrostrictive elements are thus opposite in the direction of electric distortion. For example, when the lower element expands, the upper element retracts.
Accordingly, the paired piezoelectric/electrostrictive elements exhibit electric distortion equal to a sum of the expansion of the lower element and the retraction of the upper element and its generating energy for ejection of the ink is greater than the uni-morphous type piezoelectric/electrostrictive element. As the direction of polarization in this case is yet in parallel with the direction of the electric field, the crosswise displacement of the piezoelectric/electrostrictive elements only is utilized and its efficiency will remain low. Unfortunately, the bi-morphous structure is more elaborate than the uni-morphous structure. It is also lower in the high-speed driving action than the uni-morphous structure.
The conventional uni-morphous type piezoelectric/electrostrictive driver allows the direction of polarization of the piezoelectric/electrostrictive element to is in parallel with the direction of the electric field and utilizes the displacement orthogonal to the direction. The displacement is as low in the efficiency as about ⅓ the displacement generated by the combined effect of the electric field and the polarization. While the bi-morphous structure exhibits a greater displacement than the uni-morphous structure, it is more elaborate in the arrangement. As the displacement in the direction orthogonal the direction of the electric field and the polarization is identical to that of the uni-morphous structure, its efficiency will remain low. Above all, the bi-morphous structure is unfavorable for the high-speed driving action.
The other major requirement for the ink jet head is the generation of a droplet of ink favorably controlled in the size, the timing, and the location. Most ink jet printers drive the piezoelectric/electrostrictive element to deflect the oscillator plate towards the ink pressurizing chamber and eject a droplet of the ink from its outlet. Before ejected out from an ink discharge opening 154, the ink travels in the form of a cylindrical column shape to the ink discharge orifice 100 (FIG. 23). Upon the oscillator plate returning back to the original position, the ink chamber is negatively pressurized. This causes the column of the ink to be separated into a drop let for printing down on paper and the rest which is then drawn back. At the time, some of the ink may possibly remain about the ink discharge opening 154 as denoted at 156. The lower side of the head about the ink discharge opening 154 is flat and may easily hold the remaining ink 156 which drops down in a mist together with the droplet and accumulates about a dot of print made of the droplet. The remaining ink 156 about the ink discharge opening 154 depends on the shape of the ink pressurizing chamber and the discharge orifice, the pressure in the ink pressurizing chamber, and the timing of de-pressurization. For minimizing the remaining ink, the designing of the shape and a control system for the oscillating action may undesirably be limited.
In addition, the separation of a droplet from the cylindrical column of ink forced by the oscillation of the oscillator plate 132 to run through the ink discharge opening 154 shown in FIG. 22 may significantly be controlled by the frequency and the amplitude of the oscillation. When the oscillation is uniform, the viscosity, the surface tension, the affinity to the contact surface of an inner wall of the ink discharge orifice, and other properties of the ink may also affect the separation. For example, since the ink is a non-Newton fluid, the stroke, speed, and timing of its traveling as the cylindrical column through the ink discharge opening 154 can not perfectly match the oscillation of the oscillator plate. The degree of matching may vary depending on the types of the ink. The viscosity of a commonly used ink for ink jet printing ranges from 0.7 cp to 20 cp.
The surface tension may vary depending on the type and the amount of a coloring agent, the presence or absence, the type, and the amount of a surface-active agent for stabilizing the dispersion of the coloring agent, the type and the amount of an ink solvent, and so on. For example, if the surface-active agent is used abundant for decreasing the surface tension, it may result in the blotting of the ink on paper. Although the surface-active agent is used not abundant, the surface tension of a commonly used ink for ink jet printing is as low as ranging substantially from 10 dyne/cm to 100 dyne/cm at 25xc2x0 C. The materials of the conventional ink jet heads are commonly metallic or ceramic materials. The metallic materials and the ceramic materials are high in the free energy on surface. When the material comes into direct contact with the ink, its free energy on the surface may greatly be changed thus exhibiting higher wet affinity with the ink.
This causes the print of a letter or a line particularly in monochrome printing to appear blur at the edge, as the ink is blown out in a mist. In color printing, resultant color may be tanned.
Also, disclosed in Publication (Heisei)8-238763 is an ink jet head comprising an ink pressurizing chamber 104 provided on the back with an oscillator plate 132 which is oscillated by a piezoelectric driver having a piezoelectric element 146 sandwiched between two electrodes 142 and 144, an ink feed outlet 150 for feeding ink from an ink feed chamber 116 to the ink pressurizing chamber 104, and an ink discharge orifice 100 for ejecting the ink from the ink pressurizing chamber 104.
This ink head allows the piezoelectric driver 148 to drive and deflect the oscillator plate 132 toward the ink pressurizing chamber 104 and apply a pressure into the ink pressurizing chamber 104 which in turn ejects the ink from the ink discharge orifice 100. Upon the oscillator 132 returning back to its original, the ink pressurizing chamber 104 draws a fresh supply of the ink from the ink feed chamber 116 and simultaneously takes back the rest of the ink from an ink discharge opening 154. The lower side of the head about the ink discharge opening 154 is flat and, as shown in FIG. 23, may possibly hold the remaining of the ink 3 which drops down in a mist. Also, during the piercing process for the ink discharge opening 154, burs may be developed about the ink discharge opening 154. Such burs are hardly uniform and may assist the remaining of the ink to stay about the opening and fail to prevent the ink from dropping down in a mist. Accordingly, the remaining of the ink about the opening will hardly be avoided. In addition, as a number of the heads are commonly used in a printer, their ink jet actions may be varied by the above artifacts and fail to produce uniform droplets of the ink.
It is thus an object of the present invention, in view of the above aspects of the prior art, to provide an ink jet head which comprises an ink chamber, an ink discharge passage, and an ink discharge orifice arranged in an integral assembly of ink head material layers exhibiting higher sealing effects and having no steps or undulation in the inner wall which may interrupt the propagation of a shock wave thus to transfer the pressure for discharge applied to the ink to the ink discharge opening at optimum efficiency, as well as comprising an ink feed passage and an ink feed orifice for smooth feeding of the ink, whereby blockage with the ink will be minimized and interference with neighbor heads arranged in an array will be avoided. Its another object is to provide a method of fabricating the ink jet head.
It is a further object of the present invention, in view of the above aspects of the prior art, to provide an ink jet head equipped with an ink jet driver having a uni-morphous type piezoelectric/electrostrictive element which is highly efficient with an applied electric field, simple in the construction, and adapted for high-speed driving action.
It is a still further object of the present invention, in view of the above aspects of the prior art, to provide an ink jet head which can prevent ink from creeping around and remaining about the ink discharge opening on the lower paper side of the head during the ejection of the ink and thus dropping down in a mist and can eliminate generation of burs about the ink discharge opening to produce uniform droplets of the ink consistently and thus improve the quality of printed letters and images and to provide a method of forming the ink discharge opening in a ceramic or metallic material.
The above objects can be achieved by:
(1) an ink jet head for applying a voltage to a piezoelectric element located on at least one side of an ink pressurizing chamber thus to actuate an ink jet driver, which includes a piezoelectric/electrostrictive member having a piezoelectric/electrostrictive element combined with a lower electrode plate, for pressing and ejecting ink in the form of droplets from the ink discharge opening of an ink discharge orifice, said ink pressurizing chamber communicated with the ink discharge orifice and an ink feed orifice which communicates via an ink feed passage to an ink feed chamber, wherein an ink discharge passage extending from the ink pressurizing chamber to the ink discharge orifice is continuously varied in the size of the cross section;
(2) an ink jet head defined in the paragraph (1), wherein the ink jet driver comprises a uni-morphous type piezoelectric/electrostrictive member, which has an upper electrode plate, a piezoelectric/electrostrictive element, and a lower electrode plate arranged one over the other, located on at least one side of an oscillator plate of a metallic or ceramic material, and the polarization of the piezoelectric/electrostrictive element is at an angle to the surfaces of the electrodes;
(3) an ink jet head defined in the paragraph (1), wherein the tip of the ink jet orifice projects uniformly to a height of not smaller than one micrometer;
(4) an ink jet head defined in the paragraph (1), wherein the ink feed orifice and the ink feed passage communicating from the ink feed orifice become wider in the cross section towards the ink feed chamber, the ink feed passage between the ink pressurizing chamber and the ink feed chamber is bent at least once, and the ink feed orifice is arranged not to face the forward direction of the shock wave propagated from the piezoelectric element across the ink throughout the ink pressurizing chamber;
(5) an ink jet head defined in the paragraph (1), wherein the ink jet driver comprises a uni-morphous type piezoelectric/electrostrictive member, which has an upper electrode plate, a piezoelectric/electrostrictive element, and a lower electrode plate arranged one over the other, located on at least one side of an oscillator plate of a metallic or ceramic material, and when a voltage is applied between the upper electrode and the lower electrode at different, positive and negative, polarities, the piezoelectric/electrostrictive member is deflected in two, upward and downward, directions from its original horizontal position at the non-application of voltage;
(6) an ink jet head defined in the paragraph (1), wherein the ink jet driver comprises a uni-morphous type piezoelectric/electrostrictive member, which has an upper electrode plate, a piezoelectric/electrostrictive element, and a lower electrode plate arranged one over the other, located on at least one side of an oscillator plate of a metallic or ceramic material, and when a voltage is applied between the upper electrode and the lower electrode at different, positive and negative, polarities, the piezoelectric/electrostrictive member is deflected in two, upward and downward, directions from its original horizontal position at the non-application of voltage so that the ink pressurizing chamber is pressurized by the downward deflection and de-pressurized by the upward deflection thus to perform an ejection and a drawing of the ink respectively;
(7) an ink jet head defined in the paragraph (1), wherein the tip of the ink jet orifice projects uniformly to a height of not smaller than one micrometer and also a recess having a depth of not smaller than one micrometer is provided about the ink discharge opening in the lower paper side of the head;
(8) an ink jet head defined in the paragraph (1), wherein the ink jet driver has a ratio, tp/ts, of the thickness tp of the piezoelectric/electrostrictive element to the thickness ts of the oscillator plate is within a range from 0.3 to 0.7; and
(9) a method of fabricating an ink jet head characterized by punching a ceramic material placed over a material, which is higher in the plastic deformation than the ceramic material, to form a projection and a recess about the opening on the ceramic material and after the punching process, sintering the ceramic material to remove the material placed below the ceramic material.
The present invention also covers:
(10) an ink jet head defined in the paragraph (1), wherein the inner wall of the ink discharge passage communicated to the ink discharge orifice is as smooth as not to have undulations for developing reflections of the shock wave in a reverse of the forward direction;
(11) an ink jet head defined in the paragraph (1) or (10), wherein the piezoelectric element remains supplied with a voltage, which is smaller than that for the printing action, for oscillating the ink during the non-printing mode;
(12) an ink jet head defined in any of the paragraphs (1), (10), and (11), wherein Lxe2x89xa6Mxe2x89xa64/3L is established assuming that L is the minimum diameter of the cross section in the ink discharge passage extending from the ink chamber to the ink discharge orifice or at the ink discharge orifice and M is the inner diameter of the tip of the ink discharge orifice; and
(13) an ink jet head defined in any of the paragraphs (1), (10), (11), and (12), wherein the ceramic material is made by forming layers of ceramic or glass paste one over the other in corresponding molds of not smaller than 5 micrometers in depth and sintering the layers together.
The present invention also covers:
(14) an ink jet head defined in the paragraph (4), wherein the ink feed orifice and the ink feed passage for communicating the ink feed chamber with the ink pressurizing chamber are arranged becoming wider in the cross section from the ink pressurizing chamber towards the ink feed chamber, the ink feed passage is bent at least once between the ink feed chamber and the ink pressurizing chamber, and the ink feed orifice is arranged not to face the forward direction of the shock wave propagated from the piezoelectric element across the ink throughout the ink pressurizing chamber;
(15) an ink jet head defined in the paragraph (14), wherein the ink pressurizing chamber is arranged to have a curved surface of not smaller than 5 micrometers in radius at each corner where two orthogonal walls meet each other;
(16) an ink jet head defined in the paragraph (14) or (15), wherein the ink pressurizing chamber has a wall thereof opposite to the piezoelectric element arranged not to face the forward direction of the shock wave generated by the piezoelectric element and propagated across the ink throughout the ink pressurizing chamber;
(17) an ink jet head defined in any of the paragraphs (14) to (16), wherein a slit is provided in the partition of an ink pressurizing chamber block between any two adjacent ink jet heads in the array; and
(18) an ink jet head defined in any of the paragraphs (14) to (17), wherein the ceramic material is made by forming layers of ceramic or glass paste one over the other in corresponding molds of not smaller than 5 micrometers in depth and sintering the layers together.